1. Technical Field
The present invention relates to a boiling water nuclear plant, and a steam dryer applied to the boiling water nuclear plant.
2. Background Art
A configuration example of the boiling water nuclear plant is described. A core, a steam separator, and a steam dryer are disposed in a reactor pressure vessel included in the boiling water nuclear plant. The steam separator disposed above the core separates steam generated at the core from cooling water, and the steam dryer disposed above the steam separator separates droplets from the steam separated by the steam separator. The steam dried by the steam dryer so as to contain not more than a specified amount of the droplets is supplied from a main steam nozzle to a steam turbine through a main steam line.
In the boiling water nuclear plant constructed as described above, radioactive nitrogen (N-16) is generated at the core due to reaction between oxygen (O-16) included in reactor water and neutrons. The above N-16 has a half-life of 7.1 sec, and emits high energy gamma rays (6.129 MeV). Out of the generated N-16, N-16 that becomes highly volatile chemical forms of ammonia (NH3) and nitric oxide (NO) does not remain in the reactor water, but vaporizes and introduces to the steam turbine with the steam, thereby causing an increase in the radiation dose in the turbine system.
Recently, in the boiling water nuclear plant, hydrogen injection is conducted to reduce dissolved oxygen included in the reactor water in the reactor pressure vessel for the purpose of preventing stress corrosion cracking in the structural members of the reactor pressure vessel and reactor internal therein. However, the radiation dose rate in the turbine system is inclined to rise rapidly when the hydrogen injection amount reaches a certain value in the course of increases in the hydrogen injection amount. This is because N-16 that is dissolved in the reactor water as poorly volatile chemical forms such as nitrate ions during normal operation is reduced to be highly volatile chemical forms of NH3 and NO by the hydrogen injection, and entrained with the main steam. The upper limit of the hydrogen injection amount is set due to the rise in the radiation dose rate.
Previously, as a technology for reducing the N-16 amount that transfers to the turbine system, a method is proposed in which a catalyst is disposed between the steam separator and the steam dryer to change N-16 of a highly volatile ammonia form into poorly volatile nitrogen compounds (for example, Japanese Patent Laid-Open No. 7 (1995)-151898).
Moreover, since N-16 has a short half-life, a method is proposed in which the time at which the steam reaches the main steam nozzle from the steam dryer is delayed physically to reduce N-16 by using adsorption materials, whereby the N-16 transfer amount into the turbine system is reduced (for example, Japanese Patent Laid-Open No. 2001-147291).